The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A selective reduction catalyst (SCR) may be used in exhaust systems of diesel engines to reduce NOx emissions. NO contained in the exhaust gas undergoes a reduction reaction as the exhaust gases pass through the catalyst chamber of the SCR and is reduced into nitrogen (N2) and water (H2O). The NO reduction reaction may be expressed in the following chemical reactions:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Liquid urea, instead of gaseous ammonia (NH3), is generally used as the reductant due to toxic nature and difficulty in storage of gaseous ammonia. Liquid urea is injected upstream from the SCR and mixed with the exhaust gas. The mixture of urea and exhaust gas is absorbed onto the SCR. Urea is converted into gaseous ammonia through thermal decomposition before the reduction reaction takes place.
Urea is stored in a dedicated urea tank. The urea solution generally includes 32.5% urea and 67.5% water by weight, in part, such that a low freezing temperature can be obtained. A plurality of sensors and devices are incorporated in the urea tank to ensure that sufficient urea solution is contained in the urea tank and is in good quality for an effective NOx reduction. For example, a heating device may be required to heat the urea solution to prevent the urea solution from freezing at around 11° F. A concentration sensor may be required to monitor the concentration of the urea solution to ensure that the urea solution is not intentionally or unintentionally diluted or contaminated with other liquids. A liquid level sensor may be required to ensure that sufficient amount of urea is present in the tank. These different devices and sensors require separate control and take a significant amount of space and costs.